Fuel cell systems have, as chief components, a hydrogen generator for producing hydrogen-rich reformed gas and a fuel cell for generating electric power by utilizing hydrogen generated by the hydrogen generator.
The hydrogen generator includes a reformer and a carbon monoxide remover. The reformer uses hydrocarbon-based fuel such as city gas and LPG as a material gas and generates reformed gas containing hydrogen, methane, carbon monoxide (about 10%), carbon dioxide and steam, through a steam reforming reaction of the material gas and the water. The carbon monoxide remover removes carbon monoxide from the reformed gas, which carbon monoxide poisons the fuel cell. In cases where a polymer electrolyte fuel cell is used as the fuel cell, it is necessary to reduce the carbon monoxide concentration of the reformed gas to about 10 ppm. To this end, the carbon monoxide reducer is typically formed as a two-stage remover composed of: a shift converter for removing carbon monoxide up to about 0.5% through a CO shift reaction using a shifting catalyst; and a selective oxidizer for further removing carbon monoxide through a selective oxidization reaction using a selective oxidizing catalyst to reduce the CO concentration to about 10 ppm or less.
There have heretofore been proposed various hydrogen generators that are designed in view of miniaturization, high efficiency and improved start-up performance. In a known small-sized, high-efficiency hydrogen generator, heat exchange is performed between water and heat generated in the CO shift converter and the selective oxidation reactor which perform an exothermic reaction, in order to achieve improved heat recovery efficiency.
FIG. 7 shows one example of the hydrogen generators disclosed in Patent Document 1, which is composed of a plurality of concentric circular cylinders and includes a burner 20 located at the center thereof. A reformer 8 filled with a reforming catalyst is disposed on inner side of the plurality of circular cylinders whereas a carbon monoxide reducer (shift converter in Patent Document 1) 10 filled with a carbon monoxide removing catalyst (CO shifting catalyst in Patent Document 1) is disposed on outer side of the plurality of circular cylinders. A material gas is fed from a material gas supply port 21 to the reformer 8 through a passage 22. Water is fed from a water supply port 23 and is sent to the reformer 8 through passages 24a, 24b and then the passage 22. Herein, the passage 22 is in contact with the outer periphery of the carbon monoxide reducer 10. The material gas and water sent to the passage 22 are heated by the reaction heat of the carbon monoxide reducer 10 and then introduced into the shift convertor 8 as a mixture gas of the material gas and steam. The reformer 8 is heated by the burner 20, so that a steam reforming reaction occurs between the material gas and the steam by the action of the reforming catalyst, generating hydrogen-rich reformed gas. The reformed gas generated in the reformer 8 is sent to the carbon monoxide reducer 10 by way of a passage 25 and carbon monoxide is removed from the reformed gas through a CO shift reaction by the action of the CO shifting catalyst. The reformed gas from which carbon monoxide has been removed is taken out of an outlet port 26.
In the carbon monoxide reducer 10 serving as a shift converter, a temperature gradient, which provides an inlet temperature of about 280° C. and an outlet temperature of about 200° C., is suited for the CO shift reaction. To this end, the hydrogen generator shown in FIG. 7 has a heat insulating material 27 disposed at the inner periphery of the carbon monoxide reducer 10 and the heat insulating material 27 is varied in thickness so as to be thinner at the inlet side of the carbon monoxide reducer 10 than at the outlet side, so that the heat generated by the burner 20 is more easily transmitted at the inlet side of the carbon monoxide reducer 10 than at the outlet side. As a result, the inlet temperature and outlet temperature of the carbon monoxide reducer 10 are set to the above-noted temperatures to thereby set a proper temperature gradient in the flowing direction of the reformed gas.    Patent Document 1: Japanese Laid-Open Patent Application Publication 2003-321206